1. Field of the Invention
This invention, in general, relates to optics measurement devices and, in particular, to a real-time optic testing system for quickly performing two-dimensional image analyses on an IR test lens or the like, including the determination of its modulation transfer functions (MTFs).
2. Background of the Prior Art
Today's new technologies for designing and producing complex, high quality IR optical systems require optic measurement equipment that is sophisticated, flexible and accurate.
Often this is achieved by measuring the modulation transfer function (MTF) of an optic, a measure of the ability of an optical system to transfer various levels of detail from object to image. Performance here is measured in terms of contrast (degrees of gray), or of modulation, produced for a perfect source of that detail level. MTF is like other transfer functions used in metrology where a response (output) is related to an input. Examples of systems that can be characterized by a response function are audio equipment, mechanical vibration isolation structures, and seismometers. The Optical Transfer Function (OTF) describes the response of optical systems to known input, and consists of two components—the MTF is the magnitude of the OTF and the phase transfer function (PTF) is the phase component.
The amount of detail in an image is given by the resolution of the optical system, and is customarily specified in line pairs per millimeter (lp/mm). A line pair is one cycle of a light bar and dark bar of equal width and has a contrast of unity. Contrast is defined as:
  Contrast  =                    Im        ⁢                                                  ⁢                                                ⁢        ax            -              Im        ⁢                                  ⁢        in                            Im        ⁢                                  ⁢        ax            +              Im        ⁢                                  ⁢        i        ⁢                                  ⁢        n            where Im ax is the maximum intensity produced by an image (white) and Im in is the minimum intensity (black). MTF is a plot of contrast, measured in percent, against spatial frequency measured in lp/mm. This graph is customarily normalized to a value of 1 at zero spatial frequency (all white or black). An eye test is a common MTF measurement where an ophthalmologist determines the response of the human visual system (lens and retina) to varying levels of detail-rows of letters. In this way, a doctor determines the frequency response of a patient's visual system.
The Phase Transfer Function (PTF) is a measure of the relative phase in the image as a function of frequency. A relative phase change of 180°, for example, indicates that black and white in the image are reversed. This phenomenon occurs when the OTF becomes negative. Phase reversed images still show contrast and may have a substantial MTF.
MTF specifications are frequently used for optical designs that require repeatable test standards. Some examples are reconnaissance lenses, photographic objectives and IR systems. The MTF measurement instrument is also commonly used as a production quality control tool, since operators are not required to have a high level of optical training in order to properly test the optics.
The benefits of using MTF as a system specification are three-fold. First, in many cases, optical systems employing numerous stages (lenses, film, eye, etc.) have a system MTF equal to the product of the MTF of the individual stage. This can be described as concatenation or cascading of MTF, and allows testing at a subassembly level.
Second, MTF can be specified either at a single wavelength or over a range of wavelengths, depending upon the application. Interferometric wavefront metrology is limited to certain laser wavelengths. MTF allows full spectrum specification and testing.
The third benefit of MTF testing is that it is objective and universal. A test engineer is not required to make judgments of contrast, resolution or image quality. Therefore, under the same conditions, the polychromatic MTF of a lens can be directly compared to the polychromatic MTF of a design, or to another measurement instrument.
There are several methods for measuring MTF—discrete or continuous frequency generation, image scanning, and wavefront analysis. Recent advancements in precision mechanics and electro-optics technologies have produced many practical variations on these methods that allow efficient measurement of OTF to very high accuracy. Four major categories of instrumentation exist: frequency generation, scanning, video and interferometric methods.
The most direct test of MTF is to use an object that consists of a pattern having a single spatial frequency, imaged by a lens under test. An operator measures the contrast of the image directly. This is a discrete or single-frequency measurement. Discrete frequency measurement methods are commonplace. Examples are bar charts, the USAF 1951 resolution targets, and eye charts. A series of such tests can be used to create a graph of MTF over a range of spatial frequencies.
Various mechanisms have been developed for continuously varying the source frequencies while constantly measuring the image contrast. One example of this approach utilizes a rotating radial grating with a slit aperture as an object. A pinhole is placed in the focal plane of the lens and the light passing through it is monitored with a detector. As the grating rotates, the individual black and white bars are swept across the pinhole. By moving the grating relative to the slit, the spatial frequencies of the object can be varied. The detector output is synchronized to the rotation and is a direct measure of the MTF at the radial grating spatial frequency and its harmonics.
The obvious advantage of frequency generation methods is the fact that the output is directly measured. The major disadvantage is that these methods require the simultaneous manipulation of sources and detectors, which limits instrument flexibility.
Most commercially available MTF measurement instruments use a form of image scanning. Scanning systems operate on the principles of linear system theory—the image produced by the lens with a known input, such as an infinitesimally small pinhole, is determined and the MTF is computed from this information.
Measuring MTF with this method is the optical analogy of measuring the frequency response of an audio speaker. The image produced by a lens of an infinitely small source of light will be a blur, much as the output of a speaker with a single input audio frequency will be tonal. The qualities of the blur similarly indicate the frequency response of the lens. The spatial profile of the image is called the line spread function (LSF) if the scanning is one-dimensional, or the point spread function (PSF) for two-dimensional scanning. An LSF is commonly produced by edge-scanning an image of a point source with a mechanical obscuration (knife-edge) while monitoring the intensity throughput, and then differentiating the output. It can also be produced by using a slit source and moving a pinhole or slit. The vertical or horizontal orientation of the knife determines whether sagittal or tangential scanning is achieved. If the knife-edge possesses a right angle and is diagonally traversed across the image, it will sequentially scan in the horizontal and vertical directions, yielding both sagittal and tangential edge traces. The Fourier transform of the LSF is the one-dimensional MTF.
For a true impulse response function to be derived, the finite source size must be corrected. Through linear system theory, it can be shown that this correction consists of dividing the measured MTF by the Fourier transform of the source, such that the corrected MTF data is the quotient of the uncorrected MTF data divided by the proper correction factor at discrete frequencies.
Computer algorithms quickly correct measured MTF data for finite aperture sizes (slits, pinholes, etc.). The fully corrected data can then be compared to the theoretical performance.
Through-focus MTF mapping can be generated by remeasuring the MTF at different focus planes. The effects of spherical aberration, defocus, astigmatism, field curvature and chromatic aberration can be determined from these curves. By choosing a single spatial frequency and comparing the MTF at these focal planes, the focus for best (or balanced) performance can be determined. Very high resolution (without image magnification) can now be achieved with scanning systems equipped with precision lead screws driven by stepper motors or accurate synchronous motors.
A drawback to image scanning methods is the duration of scan. Sampling theory and the parameters of the lens under test dictate the number of data points required for a properly sampled image. Insufficient sampling can significantly affect the accuracy of the MTF. Often, a long image scan will require upwards of 30 seconds measurement time.
Video methods are subject to the same theoretical considerations as the scanning methods. Typically, a solid state array is placed at the focal plane of the lens-under-test. If a pinhole source is used, the point spread function can be directly obtained from the digitized video output. The two-dimensional OTF is obtained by directly Fourier transforming this data in two dimensions. Edge traces and line spread functions can be obtained by integrating the point-spread function. If a slit source is used, the line-spread function is obtained directly and the OTF is calculated by performing a one-dimensional Fourier transform of this. In either case, the MTF is given by the modulus of the OTF. An example of a video system is described in detail in U.S. Pat. No. 5,661,816 issued on Aug. 21, 1997 in the name of Stephen D. Fantone, et al. and entitled IMAGE ANALYSIS SYSTEM.
The advantage of video MTF measurement lies in the speed with which it can be accomplished. The MTF can be updated as quickly as the solid state array can be electronically sampled and the Fourier transform calculated. This provides a continuously updated spread function and MTF curve. Video systems are very useful for alignment of optical systems specified by MTF data. An operator can move an optical component or assembly and monitor the effects of that perturbation on the MTF.
The drawbacks of video methods are inherent in the design of electronic solid state arrays. Since detector element sizes are finite and on the order of many microns, the maximum resolvable frequency is approximately 30-80 lp/mm. This problem can be circumvented by adding an optical relay system to magnify the image onto the array. However, the relay optics must be very high quality, must have a very high numerical aperture to capture the entire output of fast lenses or systems working at high off-axis angles, and should be essentially diffraction limited to not impact the measured MTF.
Pixel to pixel crosstalk, both optical and electrical, tend to increase the apparent image size and affect the measured MTF. The MTF should be corrected for these effects. High-speed video digitizing boards commonly digitize with 8-bit precision. The illumination on the video camera must be controlled so as not to saturate pixels or cause blooming. The accuracy of the computed MTF is limited by the level of digitizing. Eight-bit video MTF systems are less accurate than conventional scanning systems. However, with the right application, video-sampling methods are valuable.
Interferometric Methods
The MTF of a system may be measured with an interferometer by one of two methods: auto-correlating the pupil function of the lens-under-test or analyzing the PSF calculated by Fourier transforming the pupil wavefront. This is very convenient for systems which are suitable for testing in an interferometer and do not exhibit significant chromatic aberrations, and whose wavefront errors do not vary substantially over the wavelength of interest. With scanning, video or discrete frequency methods, the wavelength range can be adjusted by using wide band sources and spectral filters for full polychromatic testing. Interferometers rely on monochromatic sources (i.e. lasers) so that MTF is only available at these wavelengths.
In addition, since phase measuring interferometers have limited wavefront sampling capabilities, the wavefront should be fairly well corrected. Lenses with excessive wavefront errors are difficult to measure with interferometers.
Measuring MTF in the infrared has become nearly as commonplace as measuring MTF in the visible. Most IR measurements involve physical scanning apertures very similar to visible measurement scanners in both the 3-5 μm and 8-12 μm spectral bands. With advent of inexpensive IR video cameras and blackbody sources, video analysis systems are also possible.
However, measurement in the far IR region of the spectrum (FWIR), between about 8-15 micrometers, whether using scanning or video detection, is not without problems. Despite the use of sensitive IR detectors and high temperature blackbodies, the most significant challenge facing IR measurements is the signal/noise ratio. Thermal background emissions coupled with “slow” lenses (those having a large f/#) raise noise and reduce signal levels. The choice of source (slit or pinhole) dimensions is determined by the maximum frequency of interest and the magnification of the lens under test. For example, if the magnification of the optical system is 0.1×, a 200 μm pinhole diameter or slit width will allow band-limited testing to 25 lp/rm. If frequencies beyond this limit are desired, the source area must be decreased, and the signal-to-noise will decrease by a corresponding amount.
To give a sense for the effect that background IR can have on contrast measurements, assume that it is on around 30 percent of the signal. The contrast then is about 0.5 whereas, if the background is only 5 percent, the contrast will be about 0.9. Thus, the presence of background IR in the field of view of the detector is a very serious problem for MTF calculations if not properly managed.
Accordingly, it is a principle object of the present invention to provide an IR image analysis system that minimizes IR background levels with proper thermal management.
It is another object of the present invention to provide methodology and apparatus for compensating for the presence of IR background radiation in an instrument's measurement path so that MTF and other optical properties of IR optics can accurately be determined.
It is another object of the present invention to provide an instrument for measuring the modulation transfer function of IR optics in real-time while providing a convenient display of the results.
It is another object of the present invention to provide an instrument for measuring the performance of IR optical systems by performing real-time, two-dimensional image analysis and displaying the results through the use of computer system with a graphical user interface.
Other objects of the invention will, in part, appear hereinafter and, in part, be obvious. A full understanding of the invention will be had from the detailed description when read in connection with the accompanying drawings.